Internet Engineering Task Force (IETF) Y. Lee, Ed.
Request for Comments: 7449 Huawei
Category: Informational G. Bernstein, Ed.
ISSN: 2070-1721 Grotto Networking
J. Martensson
Acreo
T. Takeda
NTT
T. Tsuritani
KDDI
O. Gonzalez de Dios
Telefonica
February 2015
Path Computation Element Communication Protocol (PCEP) Requirementsfor Wavelength Switched Optical Network (WSON)Routing and Wavelength Assignment
Abstract
This memo provides application-specific requirements for the Path
Computation Element Communication Protocol (PCEP) for the support of
Wavelength Switched Optical Networks (WSONs). Lightpath provisioning
in WSONs requires a Routing and Wavelength Assignment (RWA) process.
From a path computation perspective, wavelength assignment is the
process of determining which wavelength can be used on each hop of a
path and forms an additional routing constraint to optical light path
computation. Requirements for PCEP extensions in support of optical
impairments will be addressed in a separate document.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7449.
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RFC 7449 PCEP Requirements for WSON RWA February 20151. Introduction
[RFC4655] defines the PCE-based architecture and explains how a Path
Computation Element (PCE) may compute Label Switched Paths (LSPs) in
networks controlled by Multiprotocol Label Switching Traffic
Engineering (MPLS-TE) and Generalized MPLS (GMPLS) at the request of
Path Computation Clients (PCCs). A PCC is shown to be any network
component that makes such a request and may be, for instance, an
optical switching element within a Wavelength Division Multiplexing
(WDM) network. The PCE itself can be located anywhere within the
network; it may be within an optical switching element, a Network
Management System (NMS), or an Operational Support System (OSS), or
it may be an independent network server.
The Path Computation Element Communication Protocol (PCEP) is the
communication protocol used between a PCC and PCE; it may also be
used between cooperating PCEs. [RFC4657] sets out the common
protocol requirements for PCEP. Additional application-specific
requirements for PCEP are deferred to separate documents.
This document provides a set of application-specific PCEP
requirements for support of path computation in Wavelength Switched
Optical Networks (WSONs). WSON refers to WDM-based optical networks
in which switching is performed selectively based on the wavelength
of an optical signal.
The path in WSON is referred to as a lightpath. A lightpath may span
multiple fiber links, and the path should be assigned a wavelength
for each link.
A transparent optical network is made up of optical devices that can
switch but not convert from one wavelength to another. In a
transparent optical network, a lightpath operates on the same
wavelength across all fiber links that it traverses. In such cases,
the lightpath is said to satisfy the wavelength-continuity
constraint. Two lightpaths that share a common fiber link cannot be
assigned the same wavelength. To do otherwise would result in both
signals interfering with each other. Note that advanced additional
multiplexing techniques such as polarization-based multiplexing are
not addressed in this document since the physical-layer aspects are
not currently standardized. Therefore, assigning the proper
wavelength on a lightpath is an essential requirement in the optical
path computation process.
When a switching node has the ability to perform wavelength
conversion, the wavelength-continuity constraint can be relaxed, and
a lightpath may use different wavelengths on different links along
its path from origin to destination. It is, however, to be noted
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RFC 7449 PCEP Requirements for WSON RWA February 2015
that wavelength converters may be limited for cost reasons, while the
number of WDM channels that can be supported in a fiber is also
limited. As a WSON can be composed of network nodes that cannot
perform wavelength conversion, nodes with limited wavelength
conversion, and nodes with full wavelength conversion abilities,
wavelength assignment is an additional routing constraint to be
considered in all lightpath computations.
In this document, we first review the processes for Routing and
Wavelength Assignment (RWA) used when wavelength continuity
constraints are present and then specify requirements for PCEP to
support RWA. Requirements for optical impairments will be addressed
in a separate document.
The remainder of this document uses terminology from [RFC4655].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. WSON RWA Processes and Architecture
In [RFC6163], three alternative process architectures were given for
performing routing and wavelength assignment. These are shown
schematically in Figure 1, where R stands for Routing, WA for
Wavelength Assignment, and DWA for Distributed Wavelength Assignment.
+-------------------+
| +-------+ +--+ | +-------+ +--+ +-------+ +---+
| | R | |WA| | | R |--->|WA| | R |--->|DWA|
| +-------+ +--+ | +-------+ +--+ +-------+ +---+
| Combined | Separate Processes Separate Processes
| Process | WA performed in a
+-------------------+ distributed manner
(a) (b) (b')
Figure 1: RWA Process Alternatives
These alternatives have the following properties and impact on PCEP
requirements in this document.
(a) Combined Process (R&WA)
Path selection and wavelength assignment are performed as a
single process. The requirements for PCC-PCE interaction with a
PCE implementing such a combined RWA process are addressed in
this document.
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RFC 7449 PCEP Requirements for WSON RWA February 2015
(b) Routing Separate from Wavelength Assignment (R+WA)
The routing process furnishes one or more potential paths to the
wavelength assignment process that then performs final path
selection and wavelength assignment. The requirements for PCE-
PCE interaction with one PCE implementing the routing process
and another implementing the wavelength assignment process are
not addressed in this document.
(b') Routing and Distributed Wavelength Assignment (R+DWA)
A standard path computation (unaware of detailed wavelength
availability) takes place, and then wavelength assignment is
performed along this path in a distributed manner via signaling
(RSVP-TE). This alternative is a particular case of R+WA and
should be covered by GMPLS PCEP extensions; it does not present
new WSON-specific requirements.
The various process architectures for implementing RWA have been
reviewed above. Figure 2 shows one typical PCE-based implementation,
which is referred to as the Combined Process (R&WA). With this
architecture, the two processes of routing and wavelength assignment
are accessed via a single PCE. This architecture is the base
architecture from which the requirements are specified in this
document.
+----------------------------+
+-----+ | +-------+ +--+ |
| | | |Routing| |WA| |
| PCC |<----->| +-------+ +--+ |
| | | |
+-----+ | PCE |
+----------------------------+
Figure 2: Combined Process (R&WA) Architecture
3. Requirements
The requirements for the PCC-to-PCE interface of Figure 2 are
specified in this section.
3.1. Path Computation Type Option
A PCEP request MAY include the path computation type. This can be:
(a) Both RWA, or
(b) Routing only.
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RFC 7449 PCEP Requirements for WSON RWA February 2015
This requirement is needed to differentiate between the currently
supported routing with distributed wavelength assignment option and
combined RWA. For the distributed wavelength assignment option,
wavelength assignment will be performed at each node of the route.
3.2. RWA Processing
As discussed in Section 2, various RWA processing options should be
supported in a PCEP request/reply.
(a) When the request is an RWA path computation type, the request
MUST further include the wavelength assignment options. At
minimum, the following options should be supported:
(i) Explicit Label Control (ELC) [RFC3473]
(ii) A set of recommended labels for each hop. The PCC can
select the label based on local policy.
Note that option (ii) may also be used in R+WA or R+DWA.
(b) In case of an RWA computation type, the response MUST include
the wavelength(s) assigned to the path and an indication of
which label assignment option has been applied (ELC or label
set).
(c) In the case where a valid path is not found, the response MUST
include why the path is not found (e.g., network disconnected,
wavelength not found, both, etc.). Note that 'wavelength not
found' may include several sub-cases such as wavelength
continuity not met, unsupported FEC/Modulation type, etc.
3.3. Bulk RWA Path Request/Reply
Sending simultaneous path requests for "routing only" computation is
supported by the PCEP specification [RFC5440]. To remain consistent,
the following requirements are added.
(a) A PCEP request MUST be able to specify an option for bulk RWA
path requests. A bulk path request provides an ability to
request a number of simultaneous RWA path requests.
(b) The PCEP response MUST include the path and the assigned
wavelength for each RWA path request specified in the original
bulk request.
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RFC 7449 PCEP Requirements for WSON RWA February 20153.4. RWA Path Reoptimization Request/Reply
This section provides a number of requirements concerning RWA path
reoptimization processing in PCEP.
(a) For a reoptimization request, the request MUST provide both the
path and current wavelength to be reoptimized and MAY include
the following options:
(i) Reoptimize the path keeping the same wavelength(s)
(ii) Reoptimize wavelength(s) keeping the same path
(iii) Reoptimize allowing both the wavelength and the path to
change
(b) The corresponding response to the reoptimized request MUST
provide the reoptimized path and wavelengths even when the
request asked for the path or the wavelength to remain
unchanged.
(c) In the case that the new path is not found, the response MUST
include why the path is not found (e.g., network disconnected,
wavelength not found, both, etc.). Note that 'wavelength not
found' may include several sub-cases such as wavelength
continuity not met, unsupported FEC/Modulation type, etc.
3.5. Wavelength Range Constraint
For any RWA computation type request, the requester (PCC) MUST be
allowed to specify a restriction on the wavelengths to be used. The
requester MAY use this option to restrict the assigned wavelength for
explicit labels or label sets. This restriction may, for example,
come from the tuning ability of a laser transmitter, any optical
element, or a policy-based restriction.
Note that the requester (e.g., PCC) is not required to furnish any
range restrictions.
3.6. Wavelength Assignment Preference
In a network with wavelength conversion capabilities (e.g., sparse 3R
regenerators), a request SHOULD be able to indicate whether a single,
continuous wavelength should be allocated or not. In other words,
the requesting PCC SHOULD be able to specify the precedence of
wavelength continuity even if wavelength conversion is available.
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RFC 7449 PCEP Requirements for WSON RWA February 2015
(a) An RWA computation type request MAY include the requester
preference for random assignment, descending order, ascending
order, etc. A response SHOULD follow the requester preference
unless it conflicts with the operator's policy.
(b) A request for two or more paths MUST allow the requester to
include an option constraining the paths to have the same
wavelength(s) assigned. This is useful in the case of
protection with a single transponder (e.g., 1+1 link disjoint
paths).
3.7. Signal-Processing Capability Restriction
Signal-processing compatibility is an important constraint for
optical path computation. The signal type for an end-to-end optical
path must match at the source and at the destination.
The PCC MUST be allowed to specify the signal type at the endpoints
(i.e., at the source and at the destination). The following signal-
processing capabilities should be supported at a minimum:
o Modulation Type List
o FEC Type List
The PCC MUST also be allowed to state whether transit modification is
acceptable for the above signal-processing capabilities.
4. Manageability Considerations
Manageability of WSON RWA with PCE must address the following
considerations.
4.1. Control of Function and Policy
In addition to the parameters already listed in Section 8.1 of
[RFC5440], a PCEP implementation SHOULD allow configuring the
following PCEP session parameters on a PCC:
o The ability to send a WSON RWA request.
In addition to the parameters already listed in Section 8.1 of
[RFC5440], a PCEP implementation SHOULD allow configuring the
following PCEP session parameters on a PCE:
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RFC 7449 PCEP Requirements for WSON RWA February 2015
o The support for WSON RWA.
o The maximum number of bulk path requests associated with WSON RWA
per request message.
These parameters may be configured as default parameters for any PCEP
session the PCEP speaker participates in, or may apply to a specific
session with a given PCEP peer or a specific group of sessions with a
specific group of PCEP peers.
4.2. Information and Data Models
As this document only concerns the requirements to support WSON RWA,
no additional MIB module is defined in this document. However, the
corresponding solution document will list the information that should
be added to the PCE MIB module defined in [RFC7420].
4.3. Liveness Detection and Monitoring
This document does not define any new mechanisms that imply any new
liveness detection and monitoring requirements in addition to those
already listed in Section 8.3 of [RFC5440].
4.4. Verifying Correct Operation
This document does not define any new mechanisms that imply any new
verification requirements in addition to those already listed in
Section 8.4 of [RFC5440]4.5. Requirements on Other Protocols and Functional Components
If PCE discovery mechanisms ([RFC5089] and [RFC5088]) were to be
extended for technology-specific capabilities, advertising WSON RWA
path computation capability should be considered.
4.6. Impact on Network Operation
This document does not define any new mechanisms that imply any new
network operation requirements in addition to those already listed in
Section 8.6 of [RFC5440].
5. Security Considerations
This document has no requirement for a change to the security models
within PCEP [RFC5440]. However, the additional information
distributed in order to address the RWA problem represents a
disclosure of network capabilities that an operator may wish to keep
private. Consideration should be given to securing this information.
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